As described in Japanese Patent Laid-Open Publication No. 2004-264295, a photoelectric encoder has been devised in which, as shown in FIG. 1, a lens optical system (a telecentric optical system) 40 consisting of a lens 42 and an aperture 44 (a telecentric optical aperture) is interposed between a main scale 20 and a photoreception unit 30 made of, e.g., a photoreceptor array 34. As shown in FIG. 2, the distances a and b from the lens 42 to a scale 21 of the main scale 20 and to photoreceptors 35 on the photoreceptor array 34 can thus be adjusted for magnification setting. In FIG. 1, 10 denotes a light source, and f denotes the focal length of the lens 42.
In the photoelectric encoder using this telecentric optical system 40, an image on the main scale 20 is projected onto the photoreceptor array 34 via the lens optical system (42, 44). Here, since the aperture 44 is located in the focal position of the lens 42, the image formed on the photoreceptor array 34 can be prevented from varying in magnification even if the distance (gap) between the main scale 20 and the lens 42 changes as long as the physical relationship among the lens 42, the aperture 44, and the photoreceptor array 34 is unchanged.
Nevertheless, even with the photoelectric encoder using such a telecentric optical system 40, misalignment of the photoreceptors 35 in the gap direction can change the relationship between the distance a from the lens 42 to the main scale 20 and the distance b from the lens 42 to the photoreceptors 35 as shown in FIG. 3. This changes the magnification of the image formed on the photoreception plane 31 significantly with a sharp drop in signal intensity as shown in FIG. 4.
Besides, the signal detection efficiency in the peripheral area also drops due to lens distortion and coma aberration.
Moreover, when miniaturization is intended, the optical system requires a lens that has a shorter focal length (a smaller diameter, in the case of a general-purpose lens). To maintain the aberrations smaller, however, it is necessary (1) to use an aspherical lens or (2) to use a plurality of lenses in combination (with adjustment). There have thus been such problems as a rise in cost and additional man-hours for adjustment.